MINNEAPOLIS — The federal government has spent millions of dollars to help farmers nationwide buy greenhouse-like structures called high tunnels that can add valuable weeks and even months to their growing seasons by protecting produce from chilly temperatures.

About $13 million has gone to more than 2,400 farmers in 43 states to help pay for the low-tech tunnels that look like a cross between Quonset huts and conventional greenhouses. The structures, also known as hoop houses, have been particularly beneficial in the north, where they allow farmers to plant as much as four weeks early and keep growing later in the fall.

The U.S. Department of Agriculture touts the tunnels as environmentally friendly and a way to help meet the demand for local and sustainable produce. Experts say high tunnels employ efficient drip irrigation systems and reduce pest problems, diseases and fertilizer costs.

One of the biggest advocates is Terry Nennich, an extension educator at the University of Minnesota-Crookston, who first learned of them on a 1999 trip to Normandy in northern France. Few U.S. farmers were using high tunnels then, he said.

The French are “kind of light years ahead of us,” Nennich said. “People there are more concerned about their food and pesticides and quality and freshness. Their climate isn’t as severe in the wintertime but they don’t have a lot of heat up there in the summer.”

High tunnels typically consist of a series of hoops covered with plastic that can be rolled up on the sides to allow air circulation. Prices vary by size, but they often cost just a few thousand dollars. The USDA does not keep statistics on their use or how much produce is grown in them.

They have been a “game changer” for Earl Snell, who grows organic heirloom tomatoes in two tunnels near Skipperville in southeastern Alabama.

Snell said he now can grow the tomatoes year-round and compete with southern Florida farmers, who usually produce most of the nation’s winter tomatoes. An usual, severe freeze in Florida this year has damaged crops and pushed up produce prices.

“I got some nice looking tomatoes right now. And of course everything is up, so we’re hitting the market just perfect here,” Snell said.

To promote high tunnels, the USDA’s Natural Resources Conservation Service started a three-year pilot program through its “Know Your Farmer, Know Your Food” initiative. It provided $13 million in the fiscal year that ended last Sept. 30, and more money is available this year.

USDA Deputy Secretary Kathleen Merrigan said the program supports the agency’s mission in helping small- and mid-sized farms thrive and in encouraging sound conservation practices. She said interested farmers should contact their local NRCS offices.